Because there are several possibilities, it might be wise if this has been going on for awhile to see a medical professional -- for this it probably doesn't need to be a doctor, it can be a nurse practitioner or physician's assistant. It could be a fungal infection -- ringworm is common in that moist sweaty area. It could also be from too much friction when you're having sex, which can occur for several reasons, such as having sex too often, doing it too hard, or doing it in an area that isn't well lubricated, such as with a woman who is having some problems that is creating dryness. Again, lots of possibilities.
I agree with Pax - there are lots of possibilities.
If it helps ease your mind, it doesn't sound like an STD, but see your doctor, who can help determine if it's fungal, eczema, an allergic reaction, etc.
Let us know what happens.
I use baby oil on mine after I shower it keeps it smooth